There are billions of barrels of potential liquid hydrocarbons in heavy crude formations or reservoirs, tar or oil sands in California and other places, and oil shale basins in Wyoming and Utah that are currently unprofitable to exploit for a number of reasons. Among these reasons are the following: they will not flow at ambient conditions where they are found; they are inaccessible or are accessible only with great difficulty and/or expense; further in the case of shales the average Fisher assay is less than 20 gpt; the resource has some rich strata (20 gpt or greater) but most of it is too lean to economically mine; where the resource is rich on the average, there is not enough of the total resource to economically mine; and there is too much overburden to use the technique of lifting the overburden by blasting to produce permeability and rubbilization, such as is done in the Geokinetics process.
Several attempts have been made to extract this type of resource by true in situ combustion. Unfortunately, the results have been poor. Research has shown that it is not possible to combustion retort "smooth" shale surfaces whether in slots, holes, or chunks without the presence of some fine rubble. The amount of fine rubble needed may be as low as 5% of the total resource.
There have also been attempts to increase the in situ permeability by explosive fracturing. Even fracturing by electricity has been tried.
One promising method of recovering valuable constituents from an oil shale deposit in situ is disclosed in U.S. Pat. Nos. 4,140,180 (Bridges et al) and No. 4,144,935 (Bridges et al). This process is also disclosed in Economics of Shale Oil Production By Radio Frequency Heating by R. Mallon, report no. UCRL-52942, Lawrence Livermore Laboratory, Livermore, California, May, 1980; and in "Development of the IIT Research Institute RF Heating Process For In Situ Oil Shale/Tar Sand Fuel Extraction-An Overview", by R. Carlson, E. Blase, and T. McLendon, Fourteenth Oil Shale Symposium Proceedings, Colorado School of Mines, Golden, Colorado, April, 1981. According to this process, the oil shale is processed in situ without being rubbled or explosively fractured. Metal electrodes are inserted in a set of vertical drill holes and are energized by a group of RF oscilators. The holes bound a block of shale that is to be retorted. The electric field is developed in such a way that heating within the block is almost uniform, and heating outside of the block is very low. Retorting of the shale results in a pressure build up of the hydrocarbon fluids. The oil and gas move horizontally (parallel to bedding planes), then down the electrode holes to a collection manifold. Preferably, off-peak electric power is used from existing generating stations to operate the oscillators and to keep down the costs.
This RF heating process makes use of a basic triplate transmission line concept. This triplate line heating plate concept is adaptable to a wide variety of resource materials by careful selection of the electrode array configuration and by adjusting the RF frequency to the specific dielectric of the resource. In general, the triplate electrodes consists of rows of metal pipes inserted into holes drilled either from the surface or from drifts mined into the deposit in question. The tubular electrodes may also be useful in providing an exit path for the hydrocarbonaceous products liberated by heating.
Removal of the kerogen without combustion leaves a condition of typically 15% voids with about 3% of the organic carbon left as char. This process suffers from the very great expense of having to accurately drill the triplate array holes. In addition, the process is funamentally limited to effecting a relatively small horizontal distance. Thus, in order to fully exploit a shale resource that is spread horizontally but relatively thin vertically, an RF process other than this must be available.
The recovery of shale oil has been discussed in detail and at great length because of the greater complexities thereof; however, the invention has more immediate application to heavy crudes and tar sands because the quantity and degree of heating is much less than with oil shale. However, delivering large quantities of heat, efficiently and economically for heavy crudes and tar sands has presented great problems not fully satisfied by the prior art methods.